The aim of the research outlined in this proposal is to study the process of radiation mutagenesis in the eukaryote Saccharomyces cerevisiae, using two complementary approaches. The first is to analyze this process genetically, both by isolating and characterizing genes that function in mutagenic repair and by exactly delineating in molecular terms the mutational alterations produced in wild type or radiation sensitive strains, using analytical power of the well-defined alleles of the iso-1-cytochrome c gene. Attempts will be made to isolate mutations in genes that function in the production of only certain kinds of mutational events and to directly examine the influence of nucleotide sequence on mutational specificity and frequency. Emphasis will also be laid on understanding the relationships between the diverse phenotypes of the rad6 locus, a major gene in mutagenic repair, by isolating new alleles and non-translational suppressors. The second approach is to study the physiology of repair, making use of temperature sensitive mutations, photoreactivation, and sedimentation analysis of DNA, in order to determine whether mutagenic repair is inductible, the nature of the substrate for this mode of repair, the relation of this repair to DNA synthesis and degradation and its role in restoring integrity to newly-synthesized DNA.